Separately excited DC electric motors are often used because the speed thereof can be adjusted over a wide range by varying the supply voltage for the motor. In theory, for a given supply voltage and flux, the load characteristic of these motors is independent of the direct current which flows through them. However, it is observed in practice that the motor speed decreases linearly in response to energizing current increases. The speed decrease is due to resistive losses which occur as a result of the motor internal resistance, which is the sum of the resistances of the motor armature and the commutator windings. A formula which represents this phenomenon is: EQU E=U-RI,
where:
E=the electromotive force of the motor, PA1 U=the motor supply voltage, PA1 R=the motor internal resistance, and PA1 I=the motor armature current.
The product RI represents the motor internal resistive or voltage drop. In cases where the motor operates at a constant supply voltage, increases in the shaft speed are regulated by varying the control current I, which must increase to reduce the time required for the shaft speed to increase and decrease. Consequently, if it is desired to adjust the shaft speed quickly, a considerable resistive drop develops to greatly upset the adjustment process. This is why separately excited DC electric motors which are used under these conditions have control systems including a loss compensating arrangement which enables the required speed to be reached in the required time.
A common prior art system for controlling a separately excited DC electric motor comprises a generator for deriving a control signal for the motor speed, a power amplifier connected to drive the motor in response to the control signal to adjust the motor speed, and a feedback loop for compensating, as a function of the control signal, for motor losses. The feedback loop feeds back a signal proportional to the motor current to the input of the amplifier. To derive the signal proportional to the motor current, a small feedback resistor is placed in series with the motor armature to derive a voltage proportional to the amplitude of the motor armature current.
A compensating feedback loop of this nature has been found to be ineffective when it is desired to adjust the motor speed very rapidly, equivalent, for example, to the time required for the motor commutator to turn past one or a few bars or strips. In such a case, slots between the bars produce a very irregular signal in the feedback resistor. The irregular signal includes one or a few current surges which, even when effectively filtered, cannot be used as a feedback signal. Thus, there is only an advantage in using this kind of compensating feedback loop when the required speed changes are equivalent to the commutator turning past many bars, so that a suitable feedback signal can be obtained. If there is a small angle through which the commutator turns while the speed is changing, the compensating arrangement is ineffective.
The invention proposes a control system which employs a compensating circuit having the advantage of being effective regardless of the commutator rotation angle. The improved compensating circuit is also of a very simple construction, and dispenses with a feedback resistor which necessarily adds to the internal resistance of the motor in conventional control systems.